The invention relates to a balun suitable for use at microwave frequencies. (The term "microwave" is to be understood to include millimeter waves). A balun embodying the invention may particularly but not exclusively be suitable for use over a broad range of frequencies, such as 5:1 or more, and may particularly but not exclusively be suitable for feeding a spiral antenna.
Microwave systems formed with transmission lines usually employ unbalanced transmission lines such as microstrip, but certain components such as spiral antenna require to be fed in a balanced manner, which may be done from an unbalanced line via a balun. It may be important to ensure that no unbalanced mode of propagation exists at the balanced port of the balun; for example, the presence of such a mode in the feed to a spiral antenna results in the radiation pattern of the antenna squinting with respect to the axis of the spiral. Particularly where the balun is to be operable over a very broad frequency range (a spiral antenna may have a bandwidth as great as 10:1 or more), the balun should then provide a balanced feed in a manner which is frequency-independent in nature over its operating frequency range. It may also be desirable to provide a balun which may be compact and which may be of planar form so as, for example, to be readily compatible with a planar transmission line system.